Lattice mismatch is a factor when forming one material on another, different, material. For instance, when forming epitaxial silicon-germanium (SiGe) on a silicon substrate the crystal lattice of the silicon will serve as a template for the SiGe growth and, even though the SiGe has a larger lattice constant than silicon, the SiGe will conform to the silicon crystal lattice structure. However, this will cause strain in the SiGe layer, and that strain will increase as more SiGe is grown.
For the above-described reasons, the lattice mismatch problem remains a challenge for implementing III-V semiconductor materials in device fabrication that uses heterogeneous epitaxial layers or substrate (e.g., forming III-V epitaxial layers on a silicon substrate). To address this problem, virtual III-V on silicon substrates have been proposed where there is a gradual change in material composition starting from the silicon substrate up to a layer which has a closer matched lattice constant to a III-V material and is used for the device fabrication. For a discussion of virtual III-V substrates see, for example, E. Garcia-Tabares et al., “Integration of III-V materials on silicon substrates for multi junction solar cell applications,” 2011 Spanish Conference on Electron Devices (CDE), Feb. 8-11 2011; and U.S. patent Application Publication Number 2012/0223362 A1 filed by Belenky et al., entitled “Compound Semiconductor Device on Virtual Substrate.” The contents of each of these references are incorporated by reference as if fully set forth herein.
However, unlike silicon-on-insulator (SOI) substrates where device isolation is easily achieved by etching the SOI layer which rests on a buried oxide (or BOX), III-V devices that are made on virtual III-V on silicon substrates do not have a convenient way of achieving isolation.
Therefore, given the lattice mismatch challenges highlighted above, techniques for achieving device isolation for III-V devices, such as (but not limited to) those built on III-V on silicon virtual substrates would be desirable.